If I understand the problem correctly, I think @bobc has the best answer, and @davidsrsb gives us a reason for why we may choose this workflow.
The OP (@mollux) hasn’t told us how complex the design is - he may even have access to a schematic or other design information, though not in KiCAD format - but he wants to create a close replica of an existing, proven good, layout. Perhaps the circuit has potential pitfalls (e.g., RF or high-speed signals; high voltage; high current; noise sensitivity; complex outline or cutouts; etc) that have already been solved and @mollux doesn’t want to re-invent those wheels. (A significant problem with re-inventing somebody else’s wheel is you are never sure that you have filed off all the corners. Some wheels have corners hidden in obscure places.)
Reducing the existing design to an image file(s), then using the image as a template for placement and routing seems a very reasonable approach. This may be true even if he has a KiCAD schematic and netlist; and if he doesn’t, then creating them really should be on his to-do list.